How can you tell when it’s time to repot a house plant?

Here are some great links to help you out. I do mine every spring so they have fresh nutrient rich soil and can grow happily.

https://extension.psu.edu/repotting-houseplants

http://pistilsnursery.com/how-to-repot-indoor-plants/

http://www.guide-to-houseplants.com/repotting-house-plants.html

When the water runs right thru the pot when u water it or when your plant wilts quickly after you water it. also if you will lift the plant out of the pot and check the roots, if all you see is roots and very little dirt on the outside edge where it was close to the edge of the pot it is pot bound and time to find it a bigger pot. Before you put it in a bigger pot either loosen and or trim away some of those outside roots So that it has a little bit of a fresh start.

Indications will be drooping leaves, roots showing from the bottom,leaf drop age or discoloration

Would these suggestions also be for a Christmas Cactus?

Just pop the plant out of the pot and if all you see is roots wound around the rootball. It's time to replant.

Usually you can see tight roots at the base of the plant and the plant seems to stop growing. If you take the plant out of the pot the roots are coiled tightly around the dirt inside the planter. It is now time to break the roots up and repot the plant.

If the plant is root bound than it is time.

Some of the ways to tell is if the roots are growing out of the bottom of the pot or if the plant is failing to thrive. Best wishes!